This invention relates to the casting of metal strip by continuous casting in a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow.
The twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the production of cast strip.
During casting, the casting rolls rotate such that metal from the casting pool solidifies into shells on the casting rolls that are brought together at the nip to produce a cast strip downwardly from the nip. One of the difficulties in the past has been high frequency chatter, which should be avoided because of surface defects caused in the strip. Temperature increase as the cast strip leaves the nip, called temperature rebound, is also a concern, and can cause enlargement of the shell due to ferrostatic pressure from the casting pool resulting in ridges in the strip. Temperature rebound occurs when the center of the strip contains “mushy” material, i.e. the metal between the shells that has not solidified to be self-supporting, and the latent heat from the center material will cause the shells to reheat after leaving the casting rolls.
We have found that the defects caused by high frequency chatter and temperature rebound can be controlled by maintaining and controlling the amount of mushy material that is “swallowed” in the cast strip and subsequently cooled. Some mushy material sandwiched between the solidified shells is provided to cushion the unevenness in the growth and cooling of the shells and inhibits if not eliminates high frequency chatter and the attendant strip defects. At the same time, the amount of mushy material between the solidified shells is controlled to reduce and control the amount of temperature rebound in the cast strip. If the rebound temperature is too high, it can cause at least partial remelting of the solidified shells and defects in the strip such as ridges, and in severe circumstances, breakage of the strip where the temperature is so high as to remelt the shells. The mushy material may include molten metal and partially solidified metal, and includes all the material between the shells not sufficiently solidified to be self supporting.
To further explain, the mushy material in the strip is in communication immediately below the nip with the casting pool subject to the ferrostatic pressure. When an excess amount of mushy material is between the shells of the strip below the nip, a high temperature rebound begins to re-melt and weaken the solidified shells of the cast strip. Weakened shells may locally bulge due to the ferrostatic pressure causing local excessive strip budge and surface defects in the cast strip, and with severe weakening may cause strip breakage. Also, when an excess amount of mushy material is between the shells near the strip edges, the mushy material may enlarge the edges of the strip causing “edge bulge,” or may drip from the edges of the cast strip causing “edge droop” and “edge loss.”
This temperature rebound from reheating caused by the mushy material can also effect the microstructure of the cast strip. We have found desired properties by maintaining a consistent austenitic microstructure in the cast strip at the hot rolling mill downstream of the caster. The increased temperature from temperature rebound may re-heat the strip to a temperature forming δ-ferrite, which upon cooling returns to a finer and more variable austenite microstructure.
Compounding the reheating problem is the crown shape in the typical casting rolls. As a result, the cast strip produced downwardly from the nip between the casting rolls is, for example, between 10 and 100 micrometers thicker in the center portion of the strip than adjacent edge portions. To form such cast strip having a crown, the casting rolls may have the negative crown with a circumference smaller in a center portion of the casting rolls than the circumference adjacent the strip edges. The casting rolls may be made with the casting roll surfaces slightly hyperboloid in shape. The effect of each casting roll having a casting roll circumference that is smaller in the center portion than the circumference adjacent edge portions is the strip cast is thicker in the center than adjacent the edges. In the past, this tended to cause weakening of the solidified shells in the center portion of the strip since a thicker mushy material and attendant higher temperature would tend to cause the shells in the center portion to remelt more easily and rapidly. We have found that the resulting variable amount of mushy material between the casting rolls may provide an excess amount of mushy material at the center portion of the strip than at the edge portions of the strip resulting in undesired ridges in the cast strip.
We have found a method of compensating and controlling shell formation during casting so that the solidified shells can be thicker in the center portion of the cast strip even with a substantial casting roll crown and resulting cast strip crown. We presently disclose a method for directly controlling the shell thicknesses across the cast strip so the shells and the cast strip produced is thicker in the center portion of the strip. This in turn reduces the amount of mushy material between the casting rolls at the center portion, reducing the amount of mushy material between the shells at the center portion and controlling temperature rebound and attendant strip defects, while inhibiting high frequency chatter.
Disclosed is a method of continuously casting metal strip comprising:    (a) assembling a pair of counter-rotatable casting rolls to form a gap at a nip between the casting rolls through which thin cast strip can be cast, each having casting surfaces with a center portion of at least 60% of the width of the casting rolls, two edge portions each of up to 7% of the width of the casting rolls, and at least one intermediate portion between each edge portion and the center portion, each edge portion having an average surface roughness between 3 and 7 Ra, the center portion having an average surface roughness between 1.2 and 4.0 times the surface roughness of the edge portions, and the intermediate portions having an average surface roughness between average surface roughness of the edge portions and the center portion,    (b) assembling a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the edges of the casting rolls, and    (c) counter-rotating the casting rolls to form metal shells on the casting surfaces of the casting rolls that are brought together at the nip to deliver cast strip downwardly with varied thicknesses of the metal shells across the strip width.
In the disclosed method, the surface roughness of the center portion may be tapered across its width. For example, the taper of the surface roughness of the center portion across its width may be in stepped zones.
The surface roughness of the center portion may be tapered across its width with the middle part of the center portion at least 2 Ra below the surface roughness at outmost parts of the center portion. The edge portions may have an average surface roughness of between 5 and 7 Ra. Alternatively, the edge portions may have an average surface roughness of between 3 and 6 Ra. Alternatively or additionally, the surface roughness across each edge portion may be within 1.0 Ra.
In one alternative, the surface roughness of the center portion may be substantially similar across the width.
The surface roughness of the casting surface of the center portion of the casting rolls is varied in a range between 5 and 15 Ra. Alternatively, the surface roughness of the casting surface of the center portion of the casting rolls is varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the center portion of the casting surface. The crown shape may be provided in stepped zones.
Additionally or alternatively, the surface roughness of the casting surface over the width of the casting rolls may be varied in a range between 5 and 15 Ra. The surface roughness of the casting surface over the width of the casting rolls may be varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the width of the casting surface. The crown shape may be provided in stepped zones.
The surface roughness of the casting surface of the center portion of the casting rolls is varied to correspond to a desired variation in metal shell thickness formed for the cast strip.
The edge portion of each casting roll may be between 50 mm and 75 mm wide. Alternatively, the edge portion of each casting roll is between 25 mm and 75 mm wide.
The casting rolls may be between 450 and 650 mm in diameter.
The casting rolls may have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is such that edge portions of the cast strip are of a higher temperature than the cast strip in the center portion of the strip width.
The as-cast thickness of the cast strip may be between about 0.6 and 2.4 millimeters, and the casting pool height may be between about 125 and 225 millimeters above the nip.
In addition, an apparatus is disclosed for continuously casting metal strip comprising:    (a) a pair of counter-rotatable casting rolls each having casting surfaces with a center portion of at least 60% of the width of the casting rolls, two edge portions each of up to 7% of the width of the casting rolls, and at least one intermediate portion between each edge portion and the center portion, each edge portion having an average surface roughness between 3 and 7 Ra, the center portion having an average surface roughness between 1.2 and 4.0 times the surface roughness of the edge portions, and the intermediate portions having an average surface roughness between average surface roughness of the edge portions and the center portion, and laterally positioned to form a gap at a nip between the casting surfaces of the casting rolls through which thin cast strip can be cast,    (b) a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the edges of the casting rolls, and    (c) a drive system adapted to counter-rotate the casting rolls forming metal shells on the casting surfaces of the casting rolls on the casting surfaces of the casting rolls that are brought together at the nip to deliver cast strip downwardly with varied thicknesses of the metal shells across the strip width.
In the disclosed apparatus, the surface roughness of the center portion may be tapered across its width. For example, the taper of the surface roughness of the center portion across its width may be in stepped zones.
The surface roughness of the center portion may be tapered across its width with the middle part of the center portion at least 2 Ra below the surface roughness at outmost parts of the center portion. The edge portions may have an average surface roughness of between 5 and 7 Ra. Alternatively, the edge portions may have an average surface roughness of between 3 and 6 Ra. Alternatively or additionally, the surface roughness across each edge portion may be within 1.0 Ra.
In one alternative, the surface roughness of the center portion may be substantially similar across the width.
The surface roughness of the casting surface of the center portion of the casting rolls is varied in a range between 5 and 15 Ra. Alternatively, the surface roughness of the casting surface of the center portion of the casting rolls is varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the center portion of the casting surface. The crown shape may be provided in stepped zones.
Additionally or alternatively, the surface roughness of the casting surface over the width of the casting rolls may be varied in a range between 5 and 15 Ra. The surface roughness of the casting surface over the width of the casting rolls may be varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the width of the casting surface. The crown shape may be provided in stepped zones.
The surface roughness of the casting surface of the center portion of the casting rolls is varied to correspond to a desired variation in metal shell thickness formed for the cast strip.
The edge portion of each casting roll may be between 50 mm and 75 mm wide. Alternatively, the edge portion of each casting roll is between 25 mm and 75 mm wide.
The casting rolls may be between 450 and 650 mm in diameter.
The casting rolls may have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is such that edge portions of the cast strip are of a higher temperature than the cast strip in the center portion of the strip width.
The as-cast thickness of the cast strip may be between about 0.6 and 2.4 millimeters, and the casting pool height may be between about 125 and 225 millimeters above the nip.
Also disclosed is a method of continuously casting metal strip with reduced ridges comprising:    (a) assembling a pair of counter-rotatable casting rolls to form a gap at a nip between the casting rolls through which thin cast strip can be cast, each having casting surfaces with a center portion and edge portion, the center portion having surface roughness varied across said center portion to correspond to a desired variation in metal shell thickness across the cast strip,    (b) assembling a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the edges of the casting rolls, and    (c) counter-rotating the casting rolls to form metal shells on the casting surfaces of the casting rolls that are brought together at the nip to deliver cast strip downwardly with varied thicknesses of the metal shells across the strip width.
The surface roughness of the center portion may be tapered across its width. For example, the taper of the surface roughness of the center portion across its width may be in stepped zones.
The surface roughness of the center portion may be tapered across its width with the middle part of the center portion at least 2 Ra below the surface roughness at outmost parts of the center portion. The edge portions may have an average surface roughness of between 5 and 7 Ra. Alternatively, the edge portions may have an average surface roughness of between 3 and 6 Ra. Alternatively or additionally, the surface roughness across each edge portion may be within 1.0 Ra.
In one alternative, the surface roughness of the center portion may be substantially similar across the width.
The surface roughness of the casting surface of the center portion of the casting rolls is varied in a range between 5 and 15 Ra. Alternatively, the surface roughness of the casting surface of the center portion of the casting rolls is varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the center portion of the casting surface. The crown shape may be provided in stepped zones.
Additionally or alternatively, the surface roughness of the casting surface over the width of the casting rolls may be varied in a range between 5 and 15 Ra. The surface roughness of the casting surface over the width of the casting rolls may be varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the width of the casting surface. The crown shape may be provided in stepped zones.
The surface roughness of the casting surface of the center portion of the casting rolls is varied to correspond to a desired variation in metal shell thickness formed for the cast strip.
The edge portion of each casting roll may be between 50 mm and 75 mm wide. Alternatively, the edge portion of each casting roll is between 25 mm and 75 mm wide.
The casting rolls may be between 450 and 650 mm in diameter.
The casting rolls may have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is such that edge portions of the cast strip are of a higher temperature than the cast strip in the center portion of the strip width.
The as-cast thickness of the cast strip may be between about 0.6 and 2.4 millimeters, and the casting pool height may be between about 125 and 225 millimeters above the nip.
The apparatus for continuously casting metal strip with reduced ridges may comprise:    (a) a pair of counter-rotatable casting rolls having casting surfaces with a center portion and edge portion, the center portion having surface roughness varied across the casting surface to correspond to a desired variation in metal shell thickness across the cast strip, and laterally positioned to form a gap at a nip between the casting surfaces of the casting rolls through which thin cast strip can be cast,    (b) a metal delivery system adapted to deliver molten metal above the nip to form a casting pool supported on the casting surfaces of the casting rolls and confined at the edges of the casting rolls, and    (c) a drive system adapted to counter-rotate the casting rolls forming metal shells on the casting surfaces of the casting rolls on the casting surfaces of the casting rolls that are brought together at the nip to deliver cast strip downwardly with varied thicknesses of the metal shells across the strip width.
The surface roughness of the center portion may be tapered across its width. For example, the taper of the surface roughness of the center portion across its width may be in stepped zones.
The surface roughness of the center portion may be tapered across its width with the middle part of the center portion at least 2 Ra below the surface roughness at outmost parts of the center portion. The edge portions may have an average surface roughness of between 5 and 7 Ra. Alternatively, the edge portions may have an average surface roughness of between 3 and 6 Ra. Alternatively or additionally, the surface roughness across each edge portion may be within 1.0 Ra.
In one alternative, the surface roughness of the center portion may be substantially similar across the width.
The surface roughness of the casting surface of the center portion of the casting rolls is varied in a range between 5 and 15 Ra. Alternatively, the surface roughness of the casting surface of the center portion of the casting rolls is varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the center portion of the casting surface. The crown shape may be provided in stepped zones.
Additionally or alternatively, the surface roughness of the casting surface over the width of the casting rolls may be varied in a range between 5 and 15 Ra. The surface roughness of the casting surface over the width of the casting rolls may be varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the width of the casting surface. The crown shape may be provided in stepped zones.
The surface roughness of the casting surface of the center portion of the casting rolls is varied to correspond to a desired variation in metal shell thickness formed for the cast strip.
The edge portion of each casting roll may be between 50 mm and 75 mm wide. Alternatively, the edge portion of each casting roll is between 25 mm and 75 mm wide.
The casting rolls may be between 450 and 650 mm in diameter.
The casting rolls may have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is such that edge portions of the cast strip are of a higher temperature than the cast strip in the center portion of the strip width.
The as-cast thickness of the cast strip may be between about 0.6 and 2.4 millimeters, and the casting pool height may be between about 125 and 225 millimeters above the nip.
Also disclosed is a method of forming a surface roughness on a casting roll comprising    (a) providing a texturing apparatus adapted to deliver a particulate media in a predetermined orientation against a casting roll surface, optionally using air pressure,    (b) moving the texturing apparatus axially along the casting roll surface while rotating the casting roll,    (c) varying one or more parameters from the group consisting of the rate of translation of the texturing apparatus, the rotational speed of the casting roll, the flow rate of particulate media, and, if present, the air pressure of the texturing apparatus, as the texturing apparatus translates axially along the casting roll surface    (d) forming a surface roughness in the center portion of the casting rolls of at least 60% of the width of the casting rolls, two edge portions each of up to 7% of the width of the casting rolls, and at least one intermediate portion between each edge portion and the center portion, each edge portion having an average surface roughness between 3 and 7 Ra, the center portion having an average surface roughness between 1.2 and 4.0 times the surface roughness of the edge portions, and the intermediate portions having an average surface roughness between average surface roughness of the edge portions and the center portion.
The method may further comprise varying the nozzle angle and/or distance between texturing apparatus and casting surface as the texturing apparatus translates axially along the casting roll surface.
In one alternative, the rate of translation of the texturing apparatus axially along the casting roll may be varied between 0.25 and 4 inches per minute. The rotational speed of the casting roll may be varied between 10 and 20 revolutions per minute. The flow rate of particulate media may be varied between about 10 and 60 pounds per minute. The air pressure of the texturing apparatus may be varied between about 10 and 120 pounds per square inch.
The formed surface roughness of the center portion may be tapered across its width. For example, the taper of the surface roughness of the center portion across its width may be in stepped zones.
The surface roughness of the center portion may be tapered across its width with the middle part of the center portion at least 2 Ra below the surface roughness at outmost parts of the center portion. The edge portions may have an average surface roughness of between 5 and 7 Ra. Alternatively, the edge portions may have an average surface roughness of between 3 and 6 Ra. Alternatively or additionally, the surface roughness across each edge portion may be within 1.0 Ra.
In one alternative, the surface roughness of the center portion may be substantially similar across the width.
The surface roughness of the casting surface of the center portion of the casting rolls is varied in a range between 5 and 15 Ra. Alternatively, the surface roughness of the casting surface of the center portion of the casting rolls is varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the center portion of the casting surface. The crown shape may be provided in stepped zones.
Additionally or alternatively, the surface roughness of the casting surface over the width of the casting rolls may be varied in a range between 5 and 15 Ra. The surface roughness of the casting surface over the width of the casting rolls may be varied in stepped zones in a range between 5 and 12 Ra. In one alternative, the casting rolls have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is coordinated with variation in surface roughness across the width of the casting surface. The crown shape may be provided in stepped zones.
The surface roughness of the casting surface of the center portion of the casting rolls is varied to correspond to a desired variation in metal shell thickness formed for the cast strip.
The edge portion of each casting roll may be between 50 mm and 75 mm wide. Alternatively, the edge portion of each casting roll is between 25 mm and 75 mm wide.
The casting rolls may have a crown shape adapted to form a crown in the cast strip, and the crown shape of the casting roll surface of each casting roll is such that edge portions of the cast strip are of a higher temperature than the cast strip in the center portion of the strip width.